Grand Finale: All Out battle help

I don't like saying this, but with just this you'll probably end up failing more since you don't have ways to deal with most servant enemies in the quest, don't have CE options and don't have the suits, but I'll still answer it just for the sake of doing it.
Front line: Cu alter (Borrowed support), Waver (Volumen), Mash
Back line: Archuria (Chaldea Lifesavers or MLB White rose), Scat (Golden Sump or MLB white rose), Ozy (Kaliedoscope)
Equip either Archuria or Scat with the White Princess MLB.
Use the regular Mystic code.
Run the quest. Kill Jack first while focusing keeping support Cu Alter alive with Waver and Mash.
This is the hardest part since I'm pretty sure all of them will focus on Cu Alter and you need some RNG luck here or something.
Proceed to kill Amakusa before he becomes a problem with Cu alter of course who you are keeping alive.
Ozy appears, but kill Altria first due to her ability to crit, and this is where it turns around from the guides.
DONT KILL OZY FIRST.
This will literally fuck your team up since you are relying on evades and invincibility is rare you don't have that.
Instead Archuria appears since I assume one if your team died.
Make sure to KILL NERO FIRST.
After that if Arcuria died with the death of Nero Scat will appear if Waver is still alive then charge her NP and kill Ozy this will be hard though I suggest using scat's third skill Divine Slayer and the mystic code buffs to negate his defense buff also this would be easier if Cu alter is alive.
The time when someone dies and Medea is the only one Ozy appears and uses his NP killing her.
There you're finished.
BTW this is an assumption that all Servants you mentioned are max leveled or higher and has skills ranging from 4-7 in the least, but if they're all literally 60-75, just give up. You could still get lores for some other events.
This strategy worked for me and with some RNG luck I was able to finish it without relying on Command Seals, but it might not work for you. I'm just telling you since some people I know got fucked over by Ozy's death granting sure hit to everyone and Nero NPs (which made me hate her SO MUCH), and without me browsing some other sites and realize I could kill Nero first I would've given up.
You need to adapt to thing in the game and not blindly follow the guide.
This would literally be a jugle between evades and buffs and knowing when to do that is important also save your Mystic code attack buff to the last also use Command seals when necessary, but save it for literally the last.
Well may RNGesus or something give you luck like how Ozy NPed my Jack who had evasion on charging her NP.

Honestly your servants seem kinda underlevelled for that quest. I personally found it really difficult even with all of my five stars max ascended to at least level 80.

That said, I did use a similar team to yours to beat it. I put my own MLB white rose on Altria archer, and put her and Ozy on the backline to deal with Nero and Medea (I saved her for last cos the def buff she gives when she dies is annoying). My front row consisted of Mash, Vlad and Cu Alter support with MLB white rose. He dominates this quest until Ozy dies, after which his dodge won't be effective and he should die pretty quickly. That shouldn't be a problem because at that point, if you've done well, you should only have Nero and Medea left (kill order: Jack -> Amakusa -> Altria lancer alter -> Ozy -> Nero -> Medea, but for those last two though you could also take out Nero's first 3 guts, then kill medea before killing Nero so Medea won't get guts, up to you).

For you I'd say use Iri, Waver/Mash and Cu Alter support in frontline, Waver/Mash (the other one), MLB white rose Altria and Ozy for backrow. You can put MLB white rose on Altria and Kscope on Mash/Iri, whoever you want. Boulder could go on Ozy for use against Medea at the end. Also, since you don't have it yet, I would highly recommend doing the quest and getting the Atlas institute mystic code for this fight if you can, it was practically made for challenge quests. Use its debuff clear on the second turn on Cu Alter to clear Jack's buff down and Medea's poison.

Let Cu Alter carry you until you kill Ozy, and support him with Iri/Mash/Waver. Save Cu Alter's NP to use on enemy Ozy, he's really annoying to kill otherwise. If possible, try to use a NPBB chain on him, that should kill him. The other servants before Ozy should be fairly easy for Cu Alter to take out. After Ozy dies, Cu Alter will also die quickly, and then you can use Altria against Nero. By the time you're up against Medea, in all likelihood your own Ozy should be out too, and you can use him and Altria to finish her off.

Even with all that though, I don't know how successful this strategy will be for you since, as I said, this is what I did with servants of higher level. Good luck with it, whatever you do.

Personally i would get the atlas suit right now. I tried the default and it is just too hard.